Over the last 20 years, we have witnessed a staggering increase in childhood obesity. The overall prevalence of obesity (defined as > 95th percentile of age- and sex-specific body mass index growth charts) has increased by 5% for children aged 2-5 and 13% for children aged 6-19. According to the most recent NHANES population-based survey, almost 1 out of 5 US children aged 6 and older now meets this definition of obese. While energy balance is a key determinant of weight change, recent evidence suggests that well-known causes of obesity, including poor diet, physical inactivity, and genetic predisposition, may not fully account for the rapid increase in obesity rates. Emerging laboratory research supports the hypothesis that common environmental chemicals may negatively impact lipid regulation in favor of weight gain. However, there have been very few epidemiological studies of environmental causes of obesity. Human exposure to phthalates is ubiquitous and a number of lines of evidence point to phthalates as possible obesogens. Starting in 1998, NIEHS and EPA jointly funded a network of Children's Environmental Health and Disease Prevention Research Centers, with the mission of addressing the role of environment in children's health. The Mount Sinai, Columbia, and Cincinnati Children's Centers each enrolled independent prospective birth cohorts, collected third trimester urine specimens, and followed children up for growth and development. Each of these Centers has already measured maternal urinary phthalate metabolites in the third trimester. We propose to pool these data in order to efficiently examine the association between maternal phthalate concentrations and longitudinal measures of adiposity from birth through age 7 years. To our knowledge, these three birth cohorts together represent the only US studies with currently available data on phthalates and growth through age 7, and constitute a rich and unique resource to efficiently address the aims of this study while providing the foundation for future pooling efforts to address the impact of environmental chemicals on children's health.